The present invention relates to a process for preparing particles of a high water-absorbent resin, and more particularly to a process for preparing particles of a high water-absorbent resin which is suitably used as agents for improving water absorption used in sanitary goods such paper diapers, sanitary napkins, tampons and disposable dustclothes, water-retaining agents or soil conditioners in the fields of agriculture and horticulture, and other various uses of coagulation of sludge, prevention of dew condensation on building materials, dehydration of oil, and so on.
Water-absorbent resins have, hitherto, been used in the manufacture of paper diapers, sanitary napkins, tampons, disposable dustcloths and other sanitary goods, and as water-retaining agents or soil conditioners in the fields of agriculture and horticulture. They have been also used for the purpose of coagulation of sludge, prevention of dew condensation on building materials, dehydration of oil, and so on.
These known water-absorbent resins include crosslinked polyacrylic acid salts, hydrolyzates of crosslinked acrylic acid ester-vinyl acetate copolymers, crosslinked starch-acrylic acid salt graft copolymer, hydrolyzates of crosslinked starch-acrylonitrile graft copolymers, crosslinked polyvinyl alcohol grafted with maleic anhydride, crosslinked polyethylene oxide, and so on.
These high water-absorbent resin powders are prepared in a manner wherein a high water-absorbent polymer is prepared in reversed phase suspension polymerization, reversed phase emulsion polymerization, aqueous solution polymerization or a polymerization using an organic solvent, then the prepared polymer is dried as it is, further, when occasion demands, the dried one is pulverized.
The high water-absorbent resin powders prepared by the above-mentioned manners, however, have a good many fine particles which can pass through a 100 mesh standard sieve. Thus, such high water-absorbent resin powders have the following defects.
(1) It is easy to generate dust, the working surrounding becomes easily bad and final products easily loss in weight.
(2) The miscibility and dispersibility of the resin powder with an other material are bad.
(3) It is easy to make undissolved lumps of the resin powder when contacting with liquid.
(4) Bridging and flushing are easily caused in a hopper due to poor fluidity of the resin powder.
For solving the above-mentioned defects, it have been proposed to remove fine particles or to prepare resin particles by using a binder. The former method is economically disadvantage and the later method is in danger of catching fire in drying step of the obtained particles when using an organic binder. Also, if insufficiently drying, the final particles are problematic in safety to human bodies due to the remaining organic solvent therein. When using as the binder an aqueous liquid such as water alone, an aqueous mixture of water and an organic solvent compatible with water or an aqueous solution wherein a water-soluble high molecular weight compound is dissolved in water or the aqueous mixture as mentioned above, though there is no problem caused in the case that the organic solvent is used as the binder, it is difficult to uniformly mix or disperse the particles with or in the aqueous liquid and big lumps of the resin particles, having high density (hereinafter referred to as "block") are produced in the obtained particles because of high rate of water absorption of the particles, thus resulting in that it is difficult to finally obtain uniform particles.
As a process for preparing particles of a water-absorbent resin which improves the above-mentioned defects, there are processes described in Japanese Unexamined Patent Publication No. 61-97333 and No. 61-101536, wherein a mixture of a high water-absorbent resin powder and an inorganic powder is stirred by using a specific apparatus, and to the mixture is added the aqueous liquid containing binder. According to these processes it is required to supply the aqueous liquid in the state of a fine droplet for uniformly mixing the high water-absorbent resin particles with the aqueous liquid. When using the water-soluble high molecular weight compound as the binder, however, for supplying the aqueous liquid in the state of a fine droplet, it is necessary to make the amount of the binder small or to dissolve the binder in a large amount of water because of its high viscosity. Thus, the obtained particles are poor in strength or it is required to expend much time and much energy for drying them. Further, if the aqueous liquid is not supplied in the state of a fine droplet to the resin particles stirred at high speed by using the specific apparatus, the blocks are caused partially.
It is an object of the present invention is to provide a process for preparing particles of a high water-absorbent resin which do not have fine particles unsuitable for practical use and have high rate of water absorption.
This and other objects of the present invention will become apparent from the description hereinafter.